An Ordnance Serial Number is the serial number assigned to a piece of ordnance, such as a rifle or artillery, etc.

big guns, battery, cannon, ordnance, gunnery

artillery, ordnance, funnery, gun emplacement, cannonry

The name dates back to World War II. Much of the mapping carried out during the war was done by the Department of Ordnance (the guys who take care of munitions, artillery, tanks etc.). Their maps then, were known as Ordnance Survey maps. Today, Ordnance Survey maps are carried out by civilian teams and have nothing to do with ordnance but the name has stuck.

Answer The unit name of "4th Field Artillery Service Battery" does not appear in the list of units of my reference book. There was a unit named 4th Ordnance Service Center and others like 4th Ordnance Battalion. Reference: Dept of Army Pamphlet 672-1